The cornea is a light-sensitive tissue, according to new research. The findings add to past work proving the retina senses light as part of its role in synchronizing our body clocks to Earth’s cycle of light and darkness.

“Now we know that people have more photo sensors in their eye and body than was previously guessed, but the speculation of what comes next might be the most exciting aspect of this,” says Van Gelder, director of the University of Washington Medicine’s Eye Institute. He is a co-lead of the study published in the Proceedings of the National Academy of Sciences.

The study’s most compelling finding is that neuropsin, a protein in the retina and cornea whose function in mammals was previously unknown, can sense light. Retinas and corneas kept in tissue culture could synchronize their daily rhythms to a light-dark cycle; retinas and corneas that lacked neuropsin could not do so.

This is an enticing result because neuropsin is also expressed in the skin and other parts of the body.

“It lets us consider what other types of physiology might be linked to these photoreceptors, and how we could co-opt these to help manage diseases,” says Van Gelder, professor and chair of ophthalmology.

“For example, we don’t know exactly what triggers sun-tanning. That’s an example of a phenomenon that is light-sensitive but nobody really knows the receptor for it. We don’t know what causes light sensitivity in people with lupus and other collagen vascular diseases, or why light therapy works to treat certain skin diseases.

“Your organs may have access to knowing whether it’s light or dark outside, and adjust their metabolism appropriately.”

Within the eye, neuropsin now is the sixth working photopigment scientists have identified. Van Gelder has long used a camera analogy with patients who face vision diseases and disorders to explain how these systems work.

“The cornea and eye’s lens are like the lens of the camera, focusing light, and the retina is like the film or the sensor in the back, where the image is created. For many years people viewed the eye as if it were an old-style camera, without a light meter. The discovery of the first non-visual opsin, melanopsin (1998), identified the first light meter in the eye. Just like a light meter, melanopsin measures the brightness of light but it doesn’t contribute to the image.

“The new opsins, including neuropsin and encephalopsin, suggest there is not just one light meter in the eye but multiple light meters that serve different functions. No one would’ve guessed that 20 years ago,” he says. “Now our goal is to figure out exactly how these light meters work and what functions they control.”

Although this study’s finding spotlighted new capability of the cornea, Van Gelder says, it also suggests that the retina is more complex than was previously suspected.

“We didn’t think the retina needed another photopigment; it has five we already know about. What’s remarkable is that it doesn’t use any of those pigments to synchronize its own circadian rhythms to the light-dark cycle.”

“Figuring out why evolution found advantage in using neuropsin is a question that will engage us for the foreseeable future.”

Research assistant professor Ethan Buhr is first author of this work, which was done in collaboration with laboratories at Johns Hopkins University and Cincinnati Children’s Hospital. The National Institutes of Health supported the work.